In one type of dry low NO.sub.x combustor for a gas turbine, there is provided a combustor body including a plurality of primary fuel nozzles arranged about a central secondary fuel nozzle at one end of the combustor body, a venturi downstream from the nozzles, a combustion liner defining a reaction volume, a dilution plane for admitting dilution air, and a cooling air flow arranged about the venturi walls to cool the venturi, the cooling air flowing into the reaction volume of the combustor downstream of the venturi. Also, dilution holes are often formed in the liner of the combustor in a dilution zone for purposes of shaping the gas temperature profile exiting the combustion system and providing a region for CO burnout. In the reaction volume of the combustor, carbon monoxide (CO), an undesirable pollutant and emission from a gas turbine combustion system, reacts at high temperature with air in the system to form carbon dioxide (CO.sub.2). For example, CO will react to CO.sub.2 at a temperature above approximately 1800.degree. F. but generally not below that temperature. Typically, the hot gases of combustion flow axially in the combustor in a core flow which obtains a temperature of about 2400.degree. F. Thus, the reaction of CO to CO.sub.2 occurs in the core flow as a natural result of its elevated temperature.
Compressor discharge air is typically used as a source of cooling air for the combustor, as well as for the dilution air flow, and has a combustor inlet temperature of approximately 600.degree.-700.degree. F. The cooling air for cooling the walls of the venturi about the flame holder conventionally flows into the combustion liner in the form of an annular flow. Consequently, there is an annular region of relatively cooler air flow about the centrally located core flow of the hot gases of combustion as the gases flow toward the first-stage nozzle. Moreover, while cooling air inlet admitted through dilution holes or openings in the combustor liner beneficially reduces the exit temperature of the combustor, it typically remains in cooler regions of the flow without completely mixing with the higher temperature gases of the flow. As a consequence, there are regions or streaks in the reaction volume where the cooling and/or dilution air forms a flow region having insufficient temperature to enable the carbon monoxide to react with the oxygen in the gas flow to form the more desirable carbon dioxide emissions. In short, there is a quenching of the CO to CO.sub.2 reactions in the cooler flow because the CO in that cooler gas flow region or streak does not reach the elevated temperature necessary for the reaction to occur.